Wireless Ad Hoc and Sensor Networks

466 Wireless Ad Hoc and Sensor NetworksRi_1Rj_1 Rj_2Ri_2Reader iReader jD(i, j)FIGURE 10.1Two-reader model.provide transmission powers P i and P j to achieve their respective desiredrange without considering interference. However, because of the interferenceintroduced by each other, the actual detection range in fact decreasesto R i_2 and R j_2 , respectively.As a result of not achieving the SNR at a desired detection range owingto interference, readers must attempt to increase their transmission power.If both readers increase their powers greedily, they will eventually reachthe maximum power without achieving the desired range, on account ofincreased interferences. Further, the SNR target is not met and as a resultthe tags are not read even by those that are in range. One could solve thisproblem by operating them in mutually exclusive time slots. However, asthe number of readers increases, this strategy severely degrades eachreader’s average read time and detection range and eventually increasesreading intervals.A more appropriate solution is to balance the transmission powerbetween the two readers in order to reach the equilibrium where multiplereaders can achieve their respective read ranges. In this model, if readeri transmits at P max and reader j is off, a read range greater than the targetedvalue of R i_1 can be achieved. On the other hand, there exists a powerlevel at which reader j can transmit and still allow i to achieve read rangeR i_1 . This process can be applied in reverse to enable reader j to achieveits targeted range. Under such circumstances, the average read range ofboth readers is improved over the typical on and off cycle. Such a yieldingstrategy is required in dense reader networks in which the desired rangemay not be achieved by all the readers simultaneously. The effect of theimprovement due to this strategy will be significant in dense networks.The next section details such a decentralized strategy.